1,721,063 research outputs found
Mixture Proportioning for Durable Concrete: Challenges and Changes
No full textNumerous changes and innovations have occurred in concrete materials and technology during the last century. These changes have provided engineers with many advantages in design and construction of concrete structures. At the same time, however, the application of the new developments and changes in concrete mixture proportions have also generated new durability problems.This article is published as Shah, Surendra P., Kejin Wang, and W. Jason Weiss. "Mixture proportioning for durable concrete: challenges and changes." Concrete International 22, no. 9 (2000): 73-78. Copyright 2000, American Concrete Institute. Posted with permission
Hydration, microstructure, transport properties and durability of self-compacting concrete
Get PDFBecause of the different mix design in comparison with traditional concrete and the abscence of vibration, different characteritics can be expected for self-compacting concrete (SCC). The degradation mechanisms of a cementitious material are greatly influenced by the permeability of the material for potentially aggressive substances. As the pore structure is different for SCC in comparison with traditional vibrated concrete, some changes in durability can be noticed.
This paper is first giving an overview of hydration, microstructure and transport mechanisms of self-compacting concrete. Afterwards, available durability results are summarzied in general. In this way some more general view on durability of Self-Compacting Concrete is obtained
Full scale pumping tests on SCC: test description and results
Get PDFPumping of concrete is a daily applied process, providing the possibility of continuously filling a formwork. Reports have been created dealing with the composition and the workability of the concrete, with the maximal discharge and pressures, with the characteristics of pumps and pipes, ... On the other hand, only a very few fundamental scientific studies on this topic are available.
In case of self compacting concrete (SCC), the same rules, valid for traditional concrete (TC), are applied. On the other hand, the verification of these rules, or new rules, are not reported (yet). SCC is considered as a special case of TC, having an advantageous composition for the pumping process, which should simplify the pumping and reduce the problems.
This paper shows the results from a series of pumping tests. After a description of the equipment is provided, a set of surprising results is presented : SCC causes higher pressure losses, compared to TC. Further results will prove the existence of a less viscous layer near the wall, a temperature increase inside the concrete equivalent to the pressure loss and the importance of thixotropy
Bond strength of reinforcing bars in self-compacting concrete: experimental determination
Get PDFIn reinforced concrete structures, understanding the bond mechanism is of great importance for the design (anchorage lengths, load bearing capacity, crack width, ...). Therefore this phenomenon has widely been studied for conventional vibrated concrete. For self-compacting concrete (SCC) however few test results are available and in practice the standards for conventional vibrated concrete (CVC) are applied to self-compacting concrete as well.
To fill in this lack of knowledge and to develop adapted standards for predicting the bond of reinforcement in SCC, an experimental program has been set up. The bond strength of reinforcement bars with different diameters has been tested for 1 conventional vibrated concrete and 2 self-compacting concretes. The testing method, by means of "beam-test" specimen, was based on RILEM RC6 part 1. During testing the free end slip of the bars and the applied load were recorded. The bar diameters ranged from 12 mm tot 40 mm.
From the test results it can be seen that the maximum and characteristic bond strength of self-compacting concrete is as high as for conventional vibrated concrete, or even slightly higher. The bond strength decreases however for increasing bar diameters, and the decrease seems to be a little larger for specimen made of SCC. The slip corresponding with the maximum bond strength increases for increasing bar diameters
Influence of demoulding oil on the rheological properties of fresh SCC
Get PDFIn concrete research centers, the determination of the rheological properties of concrete is becoming a daily business, easy to perform when the proper apparatus (and staff) are available. On the building site, rheometers are still absent, although some portable rheometers have been developed. Instead, the slump (flow) test is the only one performed to characterize the "workability" of the concrete. On the other hand, the rheological properties are very important on site, especially in case of self compacting concrete (SCC), in order to know whether the concrete can provide proper filling of the formwork, which pumping pressures will be needed, how long the concrete can wait before placement, ...
The difference between laboratories and building sites is not only noticeable by the test equipment, there is also another mentality and way of thinking. In a laboratory, the scientist tries to eliminate every disturbing factor as much as possible. On site, workmen prefer to work more easily, and if necessary, they apply some "tools" to ease their jobs, sometimes not being aware of the negative consequences.
Specifically in the domain of rheological characterization of concrete and its application, there is a large difference between lab and building site. In the lab, thorough cleaning of the testing materials is obtained by washing with water, but on site, as the water availability is restricted, releasing agents are applied so that the concrete does not stick to the equipment. Although these releasing agents are applied daily, very few scientific reports have been made on their influence on the rheological properties.
This paper describes the influence of one type of demoulding oil on the rheological properties of SCC. No research has been performed on the causes of the differences between SCC with or without oil, so the only purpose of this paper is to show the resulting differences. Secondly, the results have been obtained in steady state, thixotropy and loss of workability have not been investigated explicitly
Full scale pumping tests on SCC: application of the modified Hattori-Izumi theory
Get PDFStudying the flow of fresh concrete is influenced by time dependency of the rheological properties of the concrete. This time dependency can be divided into two parts : the non-reversible part, being loss of workability and the reversible part, called thixotropy. Loss of workability can be neglected in some cases, when comparing with the effect of thixotropy, but it is advised to keep track of it, especially in case of SCC for precast industry.
Several attempts have been made to characterize the thixotropic properties of concrete, but no general test procedure is known at this moment to universally describe thixotropy. In most cases, the study of the thixotropic properties is restricted to the area of interest of the authors, mostly the variation of static yield stress. Only one theory takes into account the influence of thixotropy on both viscosity and yield stress : the Hattori-Izumi theory, modified by J.E. Wallevik. This theory will be used to provide a qualitative description of the observed phenomena occurring during pumping of SCC, but due to the large complexity of both the theory and the practical application, a quantitative approach is beyond the scope of this study
Applicability of durability requirements of the European Standard EN 206-1 to self-compacting concrete
Get PDFDurability and more specifically carbonation and chloride penetration, is of major importance for reinforced structures. Before using self-compacting concrete (SCC), the carbonation and chloride penetration has to be investigated in order to locate and prevent possible problems. In order to do so, an extended experimental programme was set up simulating real conditions. This project includes laboratory experiments on the carbonation and chloride penetration of 14 self-compacting concrete (SCC) mixtures). For the carbonation study, these concretes were stored alternately one week in a carbonation room at 20°C, 60 % R.H. and 10 % CO2 and one week immersed in water. For the chloride penetration study, cylindrical specimens with a diameter of 230 mm and a height of 70 mm were alternately immersed in a solution containing chlorides and exposed to air. One cycle took approximately 1 hour.
In Belgium, no standards are available for self-compacting concrete. For this reason, the results of this experimental study are compared with the European Standard EN 206-1 : 2001 concerning 'Concrete - Specification, performance, production and conformity' and the National Application Document NBN B15-001 : 2004 of this European Standard. Both are only applicable to traditional concrete and combine a maximum W/C ratio and a minimum cement content, both depending on the environment. An optional mimimum compressive strength is mentioned.
This comparison leads to the conclusion that self-compacting concrete meets the durability requirements prescribed for traditional concrete. Based on the experimental results, the combination of a maximum W/C ratio and a minimum cement content is also a requirement for self-compacting concrete. The requirement for a minimum compressive strength is also optional for self-compacting concrete. In case of self-compacting concrete there is always a large positive difference between the maximum and real compressive strength
Shape factors of powder-type self-compacting concrete specimens subjected to uni-axial loading
No full textWhereas rheology, pumpability, durability, etc. of self-compacting concrete (SCC) have been investigated for more than 20 years now, the mechanical properties received less attention. The past few years, this aspect is getting more and more attention. However, one of the problems encountered when comparing different studies is the use of different specimen sizes all over the world. For vibrated concrete (VC), conversion factors are defined to convert the obtained compressive strength on one specimen type to another. It has been questioned whether these factors are applicable to SCC. Therefore in this study an extensive research project was carried out to determine the shape factors of powder-type self-compacting concretes. A total of two VC and ten SCC mixture designs were selected varying in cement type, cement content, water-to-cement ratio and water-to-powder ratio. Beside cubes with sides of 100 mm, 150 mm and 200 mm, cylinders with a diameter of 100 mm and diameter 150 mm were cast and cores with a diameter of 100 mm, 80 mm and 50 mm were drilled. The experimental results are compared to existing formulas and tabulated values. The influence of the fraction of powder, the cement/powder-ratio and the water/cement-ratio seems to be insignificant. A significant difference between shape-factors of self-compacting concrete and shape factors of vibrated concrete has not been found
Influence of entrapped air on the rheology of high performance mortar
No full textHigh performance mortars are an important evolution in concrete technology. Most of them are made selfconsolidating and attain a high compressive strength. They reduce the working time and increase the freedom of architects and engineers. In the last decades a lot of research is done to find the boundaries of this new type. Mixers equipped with a vacuum pump are a new way to improve the performance. A profound research is necessary, because little is known about this technique for air content reduction. Using a rheometer and simple workability tests we investigated the influence of a reduced air content on the fresh properties. This paper reports the effect of vacuum mixing on the slump flow, V-funnel, viscosity and yield value. For ultra-high performance mortar the workability and rheology decreases due to a reduced air content, while it increases for a standard self-consolidating mortar. A low significance level was found, especially for the viscosity and yield value, due to a high scatter. In conclusion, the air content can be controlled by vacuum mixing and the workability and rheology change in an acceptable way. This technique may have a more significant influence on other properties, because an important change in microstructure was found. Further research should be done in order to confirm this
Influence of sodium gluconate on cement paste viscosity
No full textSodium gluconate is applied in concrete as a set retarder, but it also influences the fluidity of the cement paste. In this research, the viscosity of plasticized cement pastes has been investigated for different dosages of sodium gluconate (SG). Isothermal calorimetric measurements confirmed the delay in hydration kinetics due to the SG addition. Viscosity measurements showed that the increasing dosage of SG lowered the viscosity of the cement paste. The adsorption of the SG was quantified and it appeared that the lower viscosity was a result of the lower particle volume fraction due to the exclusion of water for high dosages of SG. Moreover, zeta-potential measurements were performed and it was elaborated that for a similar degree of hydration lower viscosity was achieved, due to better electrostatic repulsion. Potential network formation was discussed to explain the viscosity stabilization time
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